X-ray scanning devices have historically been used in both the medical and security industries. In security applications, X-ray scanning devices have been used to display the contents of travel bags, shipped items, and/or the like without requiring personnel to undertake the cumbersome task of unpacking and/or disassembling the item in question and subsequently re-packing and/or reassembling the item for further processing. X-ray based security systems have historically been used by airport security entities (e.g., the United States Transportation Security Administration) and common carriers (e.g., United Parcel Service of America, Federal Express, and/or the like) to detect different types of contraband that may be present in items such as baggage, shipping packages, shipping containers, and the like.
In operation, X-ray radiation is transmitted through and/or scattered from items within the baggage, packages, containers, and the like. Various systems incorporate a mesh or grid that is placed upon a conveyor belt along which the baggage, packages, containers, and the like travel during the scanning process. For particularly densely packed baggage, packages, containers, and the like, it is important that X-ray radiation emitted by an X-ray scanning device penetrate the entirety of the scanned item so as to provide a desired degree of certainty that no contraband exists there-within. Conventional mesh or grid structures have proven helpful in this regard by placing such adjacent the baggage, package, container, and the like, opposite a directional orientation of the X-ray scanning device contained within the system. In this manner, such mesh or grid structures provide a baseline indicator of penetration, for example such that if the mesh or grid is visible within a scan, the item has been sufficiently penetrated with the scan for clearance or otherwise.
Although X-ray scanning devices may facilitate the security screening process for items during processing, the physical properties of X-ray radiation and X-ray detectors may, in various circumstances, obscure objects or components visible in an item scan. In general, X-ray radiation may comprise electromagnetic waves having a wavelength between 0.01 and 10 nanometers. Such electromagnetic waves propagate from an X-ray emitter through the item to be scanned, and are collected by a detector positioned opposite the item to be scanned from the X-ray emitter, the detector comprising one or more detector elements configured to measure the intensity of the transmitted radiation (i.e., the electromagnetic wave) along a radiation ray projected from the X-ray emitter to a detector element. In various embodiments, the one or more detector elements may comprise solid-state detectors generally utilized for digital imaging. The solid-state detectors may comprise a luminescent conversion layer, for example, a scintillator (e.g., a cesium iodide scintillator) in which the radiation received by the solid-state detector causes the scintillator to generate light pulses, which may subsequently be converted into digital signals that may be transmitted to a user device and displayed via a display device.
In various circumstances, such conversion layers may maintain or trap radiation, and therefore cause “ghost” images to be created in subsequent intensity signals. Such trapping effects may be caused by, for example, incomplete charge dissipation or low induced energy levels that do not decay prior to receiving additional radiation for a subsequent scan. These residual signals from a previous image remain in the detector and are superimposed on a later generated image. Such effects may become more obvious as the time between successive images is decreased, and the time for previously trapped charge accumulation to decay is likewise decreased. Moreover, stronger electromagnetic signals received by the detector elements may require additional time for the residual electromagnetic signal to decay between images.
As an item to be scanned moves to a scanning location within an X-ray scanning device, the X-ray scanning device may cause ghosted streaks to appear in a generated image. These ghosted streaks may appear as solid lines resembling radiopaque objects present within the scanned item. Where a radiopaque bar or other thin radiopaque object is oriented at least substantially parallel to the direction of travel of the item, ghosted streaks may appear to extend the length of the radiopaque object. Such ghosted streaks may cause an operator viewing the generated image to believe that the X-ray beam penetrated completely through a radiopaque object. Therefore, the operator may erroneously determine that the scanned item is clear of any prohibited items even though a complete scan was not performed on the item.
When associating a mesh or grid structure with items to be scanned, the ghosting phenomenon described above may inadvertently cause at least a portion of the mesh or grid structure to appear visible in the created image, although the electromagnetic waves did not penetrate completely through the item. For example, ghosted streaks may appear to extend at least a portion of the grid elements in the created image and the resulting image may therefore show these ghosted streaks superimposed over items even where the electromagnetic waves did not penetrate completely through the item. Thus, the mesh or grid structure may be “ghosted” (i.e., appear) in a resulting scan image, even where the item being scanned has not, in reality, been fully (or sufficiently) penetrated to actually detect all portions of the conventional mesh or grid. Consequently, personnel viewing the created image may be led to believe that a complete scan through the entirety of an item was achieved. This “ghosting” phenomena is referred to herein as “ghosting,” “ghosting lines,” “ghost lines,” “ghost images,” “ghosted images,” “ghost radiation,” “ghost signals,” and/or “ghosted lines,” all of which as should be understood to generally and interchangeably describe this phenomena.
Historically, efforts to reduce the impact of ghosting have focused on creating improved detector elements, or incorporating complex algorithms utilized to minimize the impact of ghosting. However, such solutions are prone to errors due at least in part to electromagnetic noise and other imperfections in the received signal. For example, even where grids are used, if such are oriented in a manner that results in the grid lines thereof being parallel to the direction of travel, ghosted lines may appear, although such may contain certain distortions therein. While users could conceivably identify such distortions, the risk of a user overlooking a particular distortion remains prevalent. Thus, a need exists for improved mesh or grid structures that substantially minimize the impact of “ghosting” so as to ensure sufficient penetration of all scanned items without resorting to secondary item handling and the like.